Human noroviruses (type virus Norwalk) are the major cause of epidemic non-bacterial gastroenteritis in the world. As Category B agents, analysis of their biology is a high priority. However, these viruses have been difficult to study because they have not been cultured ex vivo. Therefore many fundamental questions remain about the biology of these viruses. In 2003 we reported the discovery of the first murine norovirus (MNV-1, Appendix 1, Karst et al., STAT1-dependent innate immunity to a Norwalk-like virus. 2003. Science. 299:1575-8). We have subsequently found that this efficient enteric virus has a surprising tropism for dendritic cells and macrophages in vivo, and used this information to develop the first tissue culture replication system for a norovirus (Appendix 2, Wobus et al., Replication of a Norovirus in cell culture reveals a tropism for dendritic cells and macrophages. PLOS Biology. E432. Epub Nov 30 2004). Using this novel system we have developed a plaque assay for MNV-1, plaque purified MNV-1, purified milligram amounts of virus, obtained the first cryo-EM image of an infectious norovirus, and proved that MNV-1 RNA is infectious, even in cells that are not permissive for MNV-1 infection. This latter result indicates that MNV-1 tropism is determined at steps in infection prior to delivery of viral RNA into the cytoplasm such as binding, entry, or uncoating. Based on these new data, there are two goals of the Specific Aims. First, we propose to define fundamental mechanisms of viral entry for a viral genus that has not been studied in detail before. Second, we seek to define whether differences in these fundamental mechanisms between cells determine MNV-1 cell tropism. The Specific Aims are: Aim 1. Define the viral events involved in MNV-1 binding, entry, and uncoating. Aim 2. Define the cellular events involved in MNV-1 binding, entry, and uncoating. Aim 3. Determine the cellular and molecular basis of MNV-1 cell tropism. The proposed studies will provide fundamental information about the binding, entry, and uncoating of a very important genus of viruses that incudes many human pathogens.